fix(testing/ci/tooling): consistent unittest, venv guidance, runnable lab commands

- #9: standardize the test chain on stdlib unittest (nothing-to-install, which
  keeps M13's claims true and its planted bug intact). Aligned M5/M14/M16 prose,
  M14 lab/test_tasks.py, and ci/gitlab starters; ruff stays the only pip install.
- #20: add venv / PEP 668 / which-python guidance to M20 (+ M14/M15 local
  installs); point MCP config at the venv's absolute python.
- #21: replace M21 Part D's empty `git diff HEAD~1` with `git log -p` (no
  .gitignore added — device preserved).
- #22: add a dependency-install step before M23's green baseline on a fresh clone.
- #23: M24 reviewer/triage now tolerate code-fence-wrapped JSON (stdlib only);
  feature.patch trap untouched.
- #28: fix M27 Part D CI snippet path (working-directory) and require the gate to
  target a varying candidate; swapped_model regression kept as the fixture.

Closes #9
Closes #20
Closes #21
Closes #22
Closes #23
Closes #28

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TfzV5QvtPDz8LJS3Pu5VLT

modules/24-assistive-agents/README.md

@@ -214,6 +214,10 @@ You're reviewing a branch that adds a `clear` command to the tasks-app. The diff
    python reviewer.py apply my-review.json
    ```
 
+   (If your assistant wrapped the JSON in a ```` ```json ```` code fence even though the prompt said
+   "JSON only," don't worry — `apply` tolerates a fenced or prose-wrapped response and reads the JSON
+   out of it.)
+
 4. **Make the human decision.** Open `feature.patch` and check the agent's headline claim: the
    `clear` branch in `cli.py` never calls `save(tlist)`, so it prints "cleared all tasks" while
    `tasks.json` is untouched — a silent no-op, the exact kind of plausibility trap Module 10 trained